The present invention relates to a process for making n-butyl esters by reacting butadiene with a carboxylic acid in the presence of a Brxc3x8nsted or Lewis acid catalyst to form the unsaturated ester which is subsequently hydrogenated to form the saturated ester.
It is known that n-butyl esters such as n-butyl acetate can be produced by a number of routes. For instance, the hydroformylation of propylene in the presence of acetic acid is a method which gives a mixture of n-butyl acetate and iso-butyl acetate. This method however requires a source of syngas which increases capital costs. An alternative method is to react ethylene with vinyl acetate in the presence of an acid catalyst followed by the hydrogenation of the resultant unsaturated ester. A further method is the reaction of ethylene with ethanol in the presence of a base catalyst to form butanol and the reaction thereof with acetic acid to form butyl acetate. In addition, all these methods rely on the use of either relatively expensive feedstocks such as ethylene and n-butanol or involve multiple reaction stages or expensive catalysts and separation stages. The acid catalysed addition of butadiene to acetic acid using ion-exchangeresin catalysts having bulky counterions to improve the reaction selectivity to two isomeric C4 butenyl acetates is disclosed in several patents viz., U.S. Pat. No. 4,450,288 (alkyl pyridinium), U.S. Pat. No. 4,450,287 (quaternary ammonium), U.S. Pat. No. 4,450,289 (quaternary phosphonium). The main objective of these patents is stated to be the production of secondary butenyl acetate. However, there is no mention in these documents of the isolation of n-but-2-enyl acetate or the production of n-butyl acetate. Butadiene is a relatively inexpensive by-product of the refining process and is a potential feedstock for making butyl esters. It is commercially available either as a purified chemical or as a constituent of a hydrocarbon cut. For example, as a constituent of a mixed C4 stream obtained from naptha stream cracking. Typically such streams contain species such as butane, 1-butene, 2-butene, isobutane and isobutene in addition to butadiene. It is advantageous that a process utilising butadiene can use such streams. However, butadiene is also in equilibrium with 4-vinyl cyclohexene, a Diels Alder dimer of butadiene. This dimer can be thermally cracked back to butadiene: 
So any process involving the use of butadiene as feedstock needs to take this reversible reaction into consideration.
EP-A-84133 describes a process for the production of unsaturated alcohols and/or esters of unsaturated alcohols. The reference describes the reaction between conjugated dienes and water or aqueous carboxylic acids. The resulting product, is a complex mixture of unsaturated isomeric alcohols and esters.
It has now been found that saturated n-butyl esters and secondary butyl esters can be synthesised without resort to either (a) the hydroformylation route from propylene or (b) the use of vinyl acetate or ethylene feedstocks in relatively simple stages.
According to a first aspect of the present invention, a process is provided for making a butyl ester from butadiene, this process comprising:
a. reacting butadiene with a saturated aliphatic monocarboxylic acid to form a mixture of n-butenyl and secondary butenyl esters,
b. separating the n-butenyl ester from the secondary butenyl ester, and
c. hydrogenating the n-butenyl ester separated in step b) in the presence of a catalyst to the corresponding n-butyl ester.
The butadiene employed in step a) may be employed in the form of a substantially pure butadiene. Alternatively, a hydrocarbon mixture comprising butadiene may be employed. In one embodiment a raw (e.g. crude or depleted) C4 stream comprising butadiene, isobutene, 1 and 2-butenes and butane is employed. Such a stream may comprise up to 60% butadiene.
The secondary butenyl ester separated in step b) may be: i) recycled to step a), ii) hydrogenated in the presence of a catalyst to produce sec-butyl ester, iii) thermally cracked to produce the starting butadiene and a saturated aliphatic monocarboxylic acid; or iv) further reacted.
A preferred embodiment of the present invention is a process for making a butyl ester from butadiene, said process comprising:
a. reacting butadiene or a hydrocarbon fraction comprising butadiene with a saturated aliphatic monocarboxylic acid to form a mixture of n-butenyl and secondary butenyl esters,
b. separating the n-butenyl ester from the secondary butenyl ester,
c. recycling the secondary butenyl ester thus recovered to step a), and
d. hydrogenating the n-butenyl ester in the presence of a catalyst to the corresponding n-butyl ester.
In the present process, the saturated, aliphatic carboxylic acid suitably has 1-6 carbon atoms and is preferably acetic acid. Thus, the present process can be readily adapted to the reaction of butadiene with acetic acid to form a mixture of n-butenyl acetate (also known as crotyl acetate) and secondary butenyl acetate, the latter being separated and preferably recycled to the initial stage and the n-butenyl acetate (crotyl acetate) being catalytically hydrogenated to form n-butyl acetate.
The reaction is suitably carried out in the liquid or mixed liquid/gas phase in the presence of a solvent. It is not essential that both reactants dissolve completely in the solvent. However, it is an advantage if the solvent chosen is such that it is suitably capable of dissolving both the reactants. Specific examples of such solvents include hydrocarbons such as decane and toluene and oxygenated solvents such as butyl acetate or excess carboxylic acid reactant and recycled higher esters such as C8 acetates recycled sec-butenyl acetate. The use of excess carboxylic acid as a reactant can be advantageous when this chemistry is used to extract butadiene from an impure stream, as it facilitates reaction at high conversion of butadiene, or in process terms high efficiency of removal of butadiene. Currently the removal or recovery of butadiene from refinery streams requires a separate processing stage.
The reactions taking place in a preferred embodiment of the invention can be represented graphically by the following equation:
n-Butyl Carboxylate by the Addition of Carboxylic Acids to Butadiene 
The reactions, and in particular, the addition reaction between butadiene and the carboxylic acid (step a), may be carried out using a homogeneous or heterogeneous catalyst. Heterogeneous catalysts may be advantageous in certain cases as they can facilitate the separation of the reaction product from the reaction mixture, and/or allow the catalyst to be conveniently separated from reaction by-products (mostly high boiling point butadiene oligomeric species). The preferred catalysts are based on strong acid ion-exchange resins (e.g. Amberlyst 15(copyright), Amberlite IR120(copyright)) with a proportion of the acidic sites exchanged with bulky counterions such as tetra-phenylphosphonium counterions. Typically these counterions account for less than 10% of the available acidic sites.
The heterogeneous catalyst phase can be a partially or fully insoluble liquid phase (e.g. acidic ionic liquids, liquid acidic polymers and partially solvated polymers) or a solid (e.g. HY zeolite, strong acid macroreticular, macronet and gel ion-exchange resins and heteropolyacids of tungsten or molybdenum which have been ion-exchanged and/or supported on a carrier material). In addition to Amberlyst 15(copyright) mentioned above, other suitable examples of heterogeneous catalysts include fluorinated ion-exchange resins like Nafion(copyright), phosphoric acid functionalised polymers, and acidic oxides such as HY zeolites.
In certain cases the activity of heterogeneous catalysts may decrease after prolonged periods of use. This may be due to blockage of active sites by butadiene oligo- and polymerisation products. In such cases, it may be advantageous to carry out the process of the present invention in homogeneous phase. Suitable homogeneous catalysts include sulphonic acids, triflic (trifluoromethanesulphonic) acid and its salts (triflates). Examples of such salts include lanthanide triflates, such as lanthanum trifluoromethanesulphonic acid salts. Suitable organic sulphonic acids include methane sulphonic acid, p-toluene sulphonic acid and sulphonated calixarenes. Heteropolyacids such as tungsten Keggin structure, strong acid ionic liquids such as those described in prior published EP-A-693088, WO 95/21872 and EP-A-558187 are also suitable.
The activity of the above mentioned heterogeneous catalysts can be modified by additives such as alkyl pyridinium, quaternary alkyl ammonium, quaternary arsonium and quaternary phosphonium compounds. These additives exchange with some of the acid sites on the support and to one skilled in the art can be introduced as a salt with a displaceable counterion e.g. halides, sulphates or carboxylates.
Levels of water may also play an important part in the activity of the catalyst. For example, water levels below 5% w/w are found to be preferable because at levels above 5% w/w the catalyst activity is significantly reduced. At levels below 0.01% w/w, however, the activity has also been found to be reduced. Consequently the water level in the reaction zone is suitably in the range from 0.01 to 5% w/w based on the carboxylic acid, preferably from 0.05 to 1% w/w.
The presence of water as a reaction adjuvant can also beneficially affect the selectivity of the catalyst. For example, when Amberlyst 15(copyright) is employed as a catalyst for the reaction between butadiene and acetic acid, the rate of reaction increases through a maximum as the concentration of water is increased. Thus, the reaction occurs at an optimum rate at a particular water concentration. Thus for the Amberlyst 15(copyright) catalysed reaction between butadiene and acetic acid, preferred water concentrations are about 0.2 to 0.5 w/w %, preferably 0.3 to 0.4 w/w %.
The reasons for this effect are not fully understood. However, without wishing to be bound by any theory, it is believed that water may have an effect on the accessibility of the active sites on the catalyst, the acidity of the catalyst and/or the hydrophilicity of the catalyst. It should be noted, however, that the effect of water on both the activity and selectivity of the catalyst may also be dependent on other factors, such as the nature of the catalyst and other reaction conditions employed.
In the process of the present invention it is also advantageous to use polymerisation inhibitors such as alkylated phenols (e.g. BHT butylated hydroxytoluene, also called 2,6-di-tert-butyl-p-cresol). Other members of this series include the Irganox(copyright) series of materials from Ciba Gigy, Lowinox(copyright) series of materials from Great Lakes Chemical Corporation, tropanol(copyright) series from ICI and t-butylcatechol, nitroxides and derivatives (e.g. di-t-butylnitroxide, and n,n-dimethyl-4-nitrosoaniline, nitric oxide), stable radicals (e.g. 2,2,6,6,-tetramethyl-piperidine-1-oxyl, 2,2,6,6,-tetramethyl-4-hydroxypiperidine-1-oxyl and 2,2,6,6,-tetramethylpyrrolidine-1-oxyl).
The relative mole ratios of butadiene to the carboxylic acid reactant in the addition reaction is suitably in the range from 5:1 to 1:50, preferably in the range from 1:1 to 1:10.
This addition reaction (step a)) is suitably carried out at a temperature in the range from 20 to 140xc2x0 C., preferably from 20 to 130xc2x0 C., more preferably, 30 to 120xc2x0 C., and most preferably 40 to 90xc2x0 C. The reaction is suitably carried out at the autogeneous reaction pressure which is determined by factors such as the reaction temperature, presence of absence of solvent, excess of reactants and impurities present in the butadiene stream. An additional pressure may be applied to the system if single fluid phase is preferred e.g. no butadiene gas phase in addition to the solvated liquid phase.
The addition reaction (step a)) may be suitably carried out in a plug flow reactor with the unused butadiene being flashed off and recycled to the reactor via a vapour liquid separator, but equally could be conducted in a slurry reactor. In the case of a plug flow reactor, the butadiene can be present partially as a separate gas phase as well as being dissolved and this would result in either a trickle bed operation or a bubble bed operation. A typical LHSV (liquid hourly space velocity=volume of liquid feed/catalyst bed volume) for the carboxylic acid is 0.1 to 20 more preferably 0.5 to 5. In the case of a slurry reactor, a continuous bleed of any deactivated catalyst can be taken. In this case it is economically advantageous to run with catalyst in a various stages of deactivation to improve the utilisation of catalyst. This may result in the total loading of catalyst (activated+deactivated) reaching high levels such as 50% w/w of the reaction charge.
Preferably, the butadiene may be added gradually to the saturated aliphatic monocarboxylic acid, for example, by multiple injection at constant pressure in a batch reactor. By adding the butadiene gradually in this manner, side reactions leading to, for example, the polymerisation of the butadiene can be minimised.
In the process, distillation is suitably used to allow separation of the reactants and products. A small amount of water azeotroping of reaction products may occur due to the low levels of water employed. However, this is minor and does not significantly effect the separation of the isomeric butenyl esters, i.e. the n-butenyl ester and secondary butenyl ester (step b)). The sec-butenyl ester can be recovered and recycled to the initial addition reaction between butadiene and the carboxylic acid (step c)). It has been found that the sec-butenyl ester under reaction conditions interconverts with butadiene, free carboxylic acid and the crotyl ester. The conversion of the sec-butenyl ester to free carboxylic acid and butadiene can be achieved by treatment in the vapour phase with an acidic support such as silica-aluminas. The use of such a separate pretreatment prior to the return to the carboxylic acid and butadiene to the addition reactor may have a beneficial effect on productivity and selectivity.
The separated n-butenyl ester stream is then passed to the catalytic hydrogenation stage (step d)) to form the n-butyl ester. It is preferable to carry out the hydrogenation step under heterogeneous conditions so that it is easy to separate the catalyst from the reaction products. The catalytic hydrogenation step is suitably carried out using one or more of the following catalysts: transition metal catalysts, typically from the later groups such as ruthenium, platinum, nickel, palladium, preferably supported on a low acidity carrier such as carbon or coating a support so that little free acidity remains. Examples include Raney nickel, supported Raney nickels, 5% ruthenium on carbon.
The preferred hydrogenation catalysts are a Raney nickel catalyst supported on carbon and a ruthenium catalyst also supported on carbon.
This hydrogenation (step d)) is suitably carried out at a temperature in the range of from 80 to 250xc2x0 C., preferably, in the range of from 120 to 200xc2x0 C. This stage can be conducted at elevated, atmospheric or sub-atmospheric pressures. The hydrogenation reaction is suitably carried at a pressure in the range from 1 barg to 100 barg, preferably from 5 to 50 barg. The hydrogenation can be carried out in slurry and flow reactors. If some carboxylic acid from the previous process stages is present, this can have a detrimental effect on some catalysts e.g. nickel catalysts can dissolve to give nickel acetates. This can limit the selection of the catalyst. A solvent is not required for this reaction. The reaction can be carried out in an all gas/vapour phase or as a two phase mixture. In the latter case, a flow reactor would be operated in either a trickle bed or a bubble bed mode. The completion of the hydrogenation of the n-butenyl esters can be determined conveniently for batch reactions by cessation of hydrogen uptake and in the case of both flow and batch reactors by sampling and analysis by methods such as Gas Chromatography and UV.
The process of the present invention has the following advantages:
i. The addition of butadiene to carboxylic acids may provide an attractive alternative to hydroformylation as a source of n-butyl esters. There is a significant feedstock cost advantage to the new process.
ii. The proposed C4 butadiene based routes have an advantage over a propene-based routes when propene feedstock costs are high.
ii. In this process, impure butadiene streams can be used and this could further reduce feedstock costs and aid in refinery integration.